Copper substrate cleaning and vapor coating method



United States Patent 3,411,938 COPPER SUBSTRATE CLEANING AND VAPORCOATING METHOD Richard H. Storck, Telford, and Alfred A. Adomines,Wayne, Pa., assignors to Sperry Rand Corporation, New York, N.Y., acorporation of Delaware No Drawing. Filed Aug. 7, 1964, Ser. No. 388,3022 Claims. (Cl. 117-50) ABSTRACT OF THE DISCLOSURE The adhesion of vapordeposited chromium or aluminum overlaid with a layer of silicon monoxideto a copper substrate is improved by, after precleaning the coppersubstrate, subjecting the copper substrate to a glow discharge in thepresence of an inert or reducing atmosphere, such as may be obtained bymeans of an argon and nitrogen or hydrogen atmosphere, respectively.After the glow discharge treatment of the copper substrate the metal tobe vapor deposited thereon, chromium or aluminum, is vapor depositedthereon followed by the vapor deposition of silicon monoxide.

This invention relates to the cleaning and coating of substrates. Moreparticularly, the invention relates to the cleaning of an easilyoxidizable substrate and the depositing of an adherent coating on thecleaned substrate.

In the cleaning of a substrate prior to the deposition of a coatingthereon, it has been the practice heretofore to employ a glow dischargein a normal atmosphere to remove contaminating substances from thesurface of the substrate, such as hydrocarbons, water vapor and adsorbedgases. It has been found, however, that if the glow discharge cleaningoperation is performed in the presence of an active gas, particularlyoxygen or water vapor, and the substrate is easily oxidizable, then thecleaning operation results in the formation of an oxide film on thesurface of the substrate. Materials deposited over the oxidizedsubstrate surface have poor adhesion to the substrate. In particular, ithas been found that if chromium or aluminum is deposited on the oxidizedsurface of a copper substrate and then overlayed with a silicon monoxidelayer, the chromium or aluminum layer and the silicon monoxide layerwill subsequently peel in a high humidity atmosphere.

Accordingly, it is an object of the present invention to achieve thecleaning of a substrate without oxidizing the surface of the substrate,so that coating material may be deposited on the cleaned substratesurface which will be strongly adherent to that surface.

It is a further object of the present invention to achieve the cleaningof a substrate by a glow discharge cleaning process in which oxidizationof the substrate surface is prevented during the glow discharge, so thata strongly adherent layer of coating material may be subsequentlydeposited on the cleaned substrate.

These objects are achieved in the present invention by carrying out thecleaning of a substrate by glow discharge in an inert atmosphere, suchas argon, or in a reducing atmosphere such as hydrogen. Briefly, theinvention comprises precleaning a substrate, for example, through theuse of solutions of ferric chloride, potassium cyanide and acetone.Next, the precleaned substrate is placed in a vacuum system which isevacuated to a low pressure to remove the major portions ofcontaminating gases, such as oxygen, water vapor and hydrocarbonfractions. Thereafter, the vacuum system is filled with dry gas, such asargon, nitrogen, or hydrogen, at a relatively high pressure (thoughbelow atmospheric pressure), which is thereafter exhausted to removestill further portions of the contaminating gases until a loweroperating pressure is reached. A glow discharge is then maintained for apredetermined period of time in the inert or reducing atmosphere toclean the substrate. Next, the vacuum system is exhausted to a very lowpressure, the glow discharge automatically extinguishing itself as thepressure is reduced. The vacuum system is maintained with inert orreducing gas at this low pressure, :and one or more layers of coatingmaterial may be deposited on the cleaned substrate.

It has been found that the present invention is eminently suitable forthe cleaning of copper substrates, for the subsequent deposition byevaporation of chromium and silicon monoxide layers. The followingexample more particularly describes the invention:

A piece of copper 1" x 2" x 0.050" was preliminarily cleaned by beingdipped in a 40% solution of ferric chloride at centigrade for 1 second,thereafter, rinsed under warm tap water and dried with filteredcompressed air. This preliminary cleaning operation, involving dippingin ferric chloride, rinsing in water and drying with air, was repeatedto provide a total of 3 preliminary cleanings.

The copper piece was next dipped in a /z% solution of potassium cyanideat room temperature for 10 seconds, thereafter rinsed under cold tapwater, rinsed under room temperature demineralized water, andsuccessively dipped in three successively cleaner acetone baths under adustfree hood.

The copper piece was next immediately placd in a holder in a vacuumsystem, and the vacuum system was evacuated to roughly 0.02 micronpressure. This exhausing of the vacuum system removed major portions ofcontaminating gases, such as oxygen, water vapor and hydrocarbonfractions.. Following this, dry argon gas was applied to the vacuumsystem until the system pressure reached roughly 1,000 microns. Next,the vacuum system was evacuated to roughly microns pressure to removemost of the inert dry argon gas and still further portions of thecontaminating gases.

Following this, the argon gas was increased in pressure to roughly 200microns in the vacuum system and maintained at this pressure through anadjustable bleed valve in the argon gas line while a roughing pumpcontinued to exhaust the system. A glow discharge was maintained forroughly 5 minutes within the vacuum system at approximately 15,000 voltsAC. The glow discharge electrode consisted of a ring having :a gappositioned roughly 2 inches below the copper piece. Following the fiveminute glow dischage cleaning operation, the supply of argon to thevacuum system was stopped, and the system was evacuated. The glowdischarge continued during the evacuation of the system until a systempressure of roughly 10 microns was reached, at which time the glowdischarge automatically extinguished itself because of lack of pressurein the system. The exhausting of the vacuum system continued until apressure of roughly 0.02 micron was reached. Evaporation of chromium ispreferably deposited in a range of 0.02 to 0.1 micron.

At a system pressure of roughly 0.1 micron, approximately 200 Angstromsof chromium were evaporated onto the cleaned copper piece. The chromiumwas evaporated from a tungsten boat positioned roughly 6 inches belowthe copper piece and containing 0.013 gram of chromium at a temperaturebetween l,300 and 1,400 centigrade. Next, a layer of silicon monoxide ofa thickness varying from 1,000 Angstroms to 100,000 Angstroms wasdeposited over the chromium layer at zero stress. The silicon monoxidewas evaporated from a Drumheller type of evaporation source positionedroughly 7 inches below the copper piece. The evaporation rate wasroughly to 200 Angstroms per second.

It was found that, for the copper piece cleaned as described above byglow discharge in the argon atmosphere and subsequently coated withlayers of chromium and silicon monoxide, the layers of chromium andsilicon monoxide were strongly adherent, even when exposed to a 100%humidity atmosphere for several weeks. It was found that, for coppersamples cleaned in the same manner as above by glow discharge, not in aninert atmosphere such as argon but in an atmosphere of air, or oxygen,the coatings had poor adhesion qualities. The coatings peeled from thesubstrates when exposed to normal atmosphere for more than a few hours.

It will be noted that the invention provides for the cleaning of asubstrate in a glow discharge maintained in an inert or reducingatmosphere which prevents the oxidation of the surface of the substrateduring the cleaning operation, thereby improving the adhesion of acoating on the cleaned substrate which is subsequently depositedthereon. The invention is most suitable for the cleaning of substrateswhich are easily oxidizable, such as copper. The invention, of course,is applicable also to other substrates. Accordingly, the inventionshould be taken to be defined not in terms of the specific example givenabove but in terms of the claims, which are set forth as follows.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. The method which comprises precleaning a copper substrate byimmersing said copper substrate in sequence in an aqueous solution offerric chloride, in an aqueous solution of potassium cyanide and inacetone, subjecting the resulting copper substrate to glow discharge inthe presence of an inert or reducing gas selected from the groupconsisting of argon, nitrogen and hydrogen to additionally clean thecopper substrate, reducing the pressure during the glow dischargetreatment operation to a low pressure sufiicient to extinguish the glowdischarge, vapor depositing on the resulting treated copper substrate alayer of a metal selected from the group consisting of chromium andaluminum and subsequently vapor depositing thereon an overlaying layerof silicon monoxide.

2. The method in accordance with claim 1 wherein said precleaningoperation includes immersing the copper substrate in a solution offerric chloride at about C. for about one second followed by rinsingwith warm tap water and drying with filtered compressed air, thereafterimmersing the copper substrate in a .5% solution of potassium cyanide atroom temperature for about 10 seconds followed by rinsing with cold tapwater and then demineralized at about room temperature and then byimmersion in an acetone bath.

References Cited UNITED STATES PATENTS 2,467,953 4/ 1949 Banecroft117-107 2,799,600 7/1957 Scott 117-106 X 2,935,369 5/1960 Mignone et al.117107 X 2,985,756 5/1961 Holland 250495 3,001,893 1/1961 Kreucher etal. .117-107 X 3,085,913 4/1963 Caswell 117-10=7 X 3,108,900 10/1963Papp 11793l 2,318,559 5/ 1943 Percival.

2,849,583 8/1958 Pritikin.

3,123,493 3/1964 Brick 117-50 3,161,946 12/1964 Birkenweil 117107 X3,192,892 7/1965 Hanson et al. 118-491 3,242,090 3/1966 Grunwald 13441 XFOREIGN PATENTS 759,694 10/1956 Great Britain.

OTHER REFERENCES Holland, Vacuum Deposition of Thin Films, John Wiley &Sons, pp. 74 to 78, 80 to 83 relied on, 1956, TS. 695H6.

Holland, The Properties of Glass Surfaces, John Wiley & Sons, pp. 325 to328 relied on, received Scientific Library, June 30, 1964, TA. 450H6.

Hiler et al., WADC Technical Report, 5988, Development of a Method toAccomplish Aluminum Deposition by Gas Plating, Appendix HI, pp. to 53relied upon.

ALFRED L. LEAVITT, Primary Examiner.

A. G. GOLIAN, Assistant Examin r.

